Scrub the porcelain with a mixture of 1 tbsp. ammonia in 1 gallon hot water using a plastic dish scrubber. This will remove any greasy residue on the surface. Rinse thoroughly.
Cut a lemon in half and dip it in 1/2 cup of borax. Scrub the porcelain with the lemon-borax mixture to remove any stains. This is particularly useful for porcelain sinks with rust stains. Rinse thoroughly.
Scrub vinegar onto any remaining stains and polish the porcelain with it. Use a soft cloth to dry the porcelain and buff it to a shine.